In polar modulation, a signal is separated into its instantaneous amplitude and phase/frequency components (rather than into the classical in-phase (I) and quadrature (Q) components), and the amplitude component and phase/frequency component are modulated independently. The amplitude component may be modulated with any suitable amplitude modulation (AM) technique, while the phase/frequency component may be modulated using an analog phase locked loop (PLL).
To allow reasonable operation, the bandwidth of the PLL may be quite small, much smaller than the actual bandwidth of the transmission signal's instantaneous phase/frequency. For example, in the case where the PLL is fed by a sigma-delta converter that has a high pass noise nature, the loop filter may be narrow enough to attenuate the sigma-delta quantization noise and the phase noise of the PLL. A pre-emphasis filter may emphasize, prior to modulation, those frequency components that would be attenuated by the PLL. The pre-emphasis filter may employ inverse filtering to the linearized response of the PLL. This inverse filtering may yield a high-order infinite impulse response (IIR), which may suffer from stability problems.
Conventional practice involves calibration mechanisms in order to accurately calibrate the PLL to the predefined pre-emphasis filter. Without calibration, there is a risk that the pre-emphasis filter will not match the inverse to the PLL closed loop transfer function, which may result in enhancement of the phase noise.
It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity.